Re: [PATCH] sched: fix group_entity's share update

[Vincent Guittot]

Gentle ping ...

Vincent

On 1 December 2016 at 17:38, Vincent Guittot <vincent.guittot@xxxxxxxxxx> wrote:
> The update of the share of a cfs_rq is done when its load_avg is updated
> but before the group_entity's load_avg has been updated for the past time
> slot. This generates wrong load_avg accounting which can be significant
> when small tasks are involved in the scheduling.
>
> Let take the example of a task TA that is dequeued of its task group TG1.
> TA was the only task in TG1 which becomes idle.
>
> We have the sequence:
>
> - dequeue_entity TA->se
>     - update_load_avg(TA->se)
>     - dequeue_entity_load_avg(TG1->cfs_rq, TA->se)
>     - account_entity_dequeue(TG1->cfs_rq, TA->se)
>           TG1->cfs_rq->load.weight = 0
>     - update_cfs_shares(TG1->cfs_rq)
>                 TG1->se->load.weight is updated with the new share of
>                 cfs_rq. TG1->se->load.weight = 0.
> - dequeue_entity TG1->se
>     - update_load_avg(TG1->se) but its weight is now null so the last time
> slot (up to a tick) will be accounted with its new weight (0 in our case)
> instead of its real weight. The last time slot is accounted as an idle one
> whereas it was a running one.
>
> If the running time of TA is short enough that no tick happens when it
> runs, all running time of TG1->se will be accounted as idle time.
>
> Instead, we should update the share of a cfs_rq (in fact the weight of its
> group entity) only after having updated the load_avg of the group_entity.
>
> update_cfs_shares() now takes the sched_entity as parameter instead of the
> cfs_rq and the weight of the group_entity is updated only once its load_avg
> has been synced with current time.
>
> Cc: <stable@xxxxxxxxxxxxxxx>
> Signed-off-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> ---
>
> I have seen the problem on tip/sched/core, v4.8 and v4.7. Previous versions
> might also have the problem but I haven't not been able to test them yet.
>
>  kernel/sched/fair.c | 27 ++++++++++++++++-----------
>  1 file changed, 16 insertions(+), 11 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 18d9e75..19092fa 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -2689,15 +2689,18 @@ static void reweight_entity(struct cfs_rq *cfs_rq, struct sched_entity *se,
>
>  static inline int throttled_hierarchy(struct cfs_rq *cfs_rq);
>
> -static void update_cfs_shares(struct cfs_rq *cfs_rq)
> +static void update_cfs_shares(struct sched_entity *se)
>  {
>         struct task_group *tg;
> -       struct sched_entity *se;
> +       struct cfs_rq *cfs_rq = group_cfs_rq(se);
>         long shares;
>
> +       if (entity_is_task(se))
> +               return;
> +
>         tg = cfs_rq->tg;
> -       se = tg->se[cpu_of(rq_of(cfs_rq))];
> -       if (!se || throttled_hierarchy(cfs_rq))
> +
> +       if (throttled_hierarchy(cfs_rq))
>                 return;
>  #ifndef CONFIG_SMP
>         if (likely(se->load.weight == tg->shares))
> @@ -2707,8 +2710,10 @@ static void update_cfs_shares(struct cfs_rq *cfs_rq)
>
>         reweight_entity(cfs_rq_of(se), se, shares);
>  }
> +
> +
>  #else /* CONFIG_FAIR_GROUP_SCHED */
> -static inline void update_cfs_shares(struct cfs_rq *cfs_rq)
> +static inline void update_cfs_shares(struct sched_entity *se)
>  {
>  }
>  #endif /* CONFIG_FAIR_GROUP_SCHED */
> @@ -3583,9 +3588,9 @@ enqueue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
>                 se->vruntime += cfs_rq->min_vruntime;
>
>         update_load_avg(se, UPDATE_TG);
> +       update_cfs_shares(se);
>         enqueue_entity_load_avg(cfs_rq, se);
>         account_entity_enqueue(cfs_rq, se);
> -       update_cfs_shares(cfs_rq);
>
>         if (flags & ENQUEUE_WAKEUP)
>                 place_entity(cfs_rq, se, 0);
> @@ -3681,7 +3686,7 @@ dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
>         /* return excess runtime on last dequeue */
>         return_cfs_rq_runtime(cfs_rq);
>
> -       update_cfs_shares(cfs_rq);
> +       update_cfs_shares(se);
>
>         /*
>          * Now advance min_vruntime if @se was the entity holding it back,
> @@ -3864,7 +3869,7 @@ entity_tick(struct cfs_rq *cfs_rq, struct sched_entity *curr, int queued)
>          * Ensure that runnable average is periodically updated.
>          */
>         update_load_avg(curr, UPDATE_TG);
> -       update_cfs_shares(cfs_rq);
> +       update_cfs_shares(curr);
>
>  #ifdef CONFIG_SCHED_HRTICK
>         /*
> @@ -4761,7 +4766,7 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
>                         break;
>
>                 update_load_avg(se, UPDATE_TG);
> -               update_cfs_shares(cfs_rq);
> +               update_cfs_shares(se);
>         }
>
>         if (!se)
> @@ -4820,7 +4825,7 @@ static void dequeue_task_fair(struct rq *rq, struct task_struct *p, int flags)
>                         break;
>
>                 update_load_avg(se, UPDATE_TG);
> -               update_cfs_shares(cfs_rq);
> +               update_cfs_shares(se);
>         }
>
>         if (!se)
> @@ -9316,7 +9321,7 @@ int sched_group_set_shares(struct task_group *tg, unsigned long shares)
>                 /* Possible calls to update_curr() need rq clock */
>                 update_rq_clock(rq);
>                 for_each_sched_entity(se)
> -                       update_cfs_shares(group_cfs_rq(se));
> +                       update_cfs_shares(se);
>                 raw_spin_unlock_irqrestore(&rq->lock, flags);
>         }
>
> --
> 2.7.4
>